1. Field of the Invention
This invention relates generally to positioning apparatus and more specifically to means providing for the controlled movement and accurate positioning of a first positioning member with respect to a second positioning member.
2. Description of the Prior Art
Positioning apparatus of the prior art have typically included a fixed member such as a platen, and a movable member, such as a head, which is independently movable with respect to the platen along X and Y axes. Thus the head can be moved and accurately positioned with respect to substantially any point on the platen.
For movement along one of the axes, the head typically includes a first drive coil which is energized by a drive signal to produce a first electromagnetic force for moving the head along the platen. Such a positioning apparatus is disclosed and claimed by Bruce A. Sawyer in his copending application Ser. No. 348,008 and by Walter E. Hinds in his copending application Ser. No. 362,119, both assigned of record to the assignee of record of the present application.
It has been found that by providing a second drive coil disposed relative to the first drive coil on the head, the second drive coil can be independently energized by a second drive signal to produce a second electromagnetic force aiding the first coil in moving the head relative to the platen. The first and second drive signals typically have a phase relationship dependent upon the relative positions of the first and second coils. In this manner the harmonics of the drive signals can be cancelled to provide an improved linear movement of the head with respect to the platen. This compensated positioning system is disclosed and claimed in application Ser. No. 438,701, filed on Feb. 1, 1974 in the name of Bernabe Nocito, which is a Continuation-in-Part of application Ser. No. 264,718, filed on June 21, 1972 in the name of Bernabe Nocito, which is presently abandoned.
The first and second drive signals have varying characteristics which provide for movement of the head relative to the platen and constant characteristics which tend to maintain the head at a fixed position with respect to the platen. When it is desired to move the head to a predetermined position on the platen, the first and second drive signals are provided with varying characteristics. When the particular point is reached, the drive signals can be changed from the varying characteristics to the constant characteristics to maintain the head at the predetermined position. Typically, the head has tended to physically ring, following abrupt transitions from the varying characteristics to the constant characteristics. The ringing of the head typically takes the form of a decaying sinusoid with a frequency dependent upon the intertial mass of the head. This decaying sinusoid may persist for as long as 200 to 500 milliseconds. The decaying oscillations of the head are particularly undesirable where the positioning apparatus is used in conjunction with a laser scriber, for example, wherein the oscillations would produce a waviness in the cut of the scriber.